It seems calpress has the same problems as all-in-one-calendar does ... it won't work with W3 Total Cache. Saving the settings results in:
PHP Fatal error: Calp_Settings::get_instance(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "Calp_Settings" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition
in wp-content/plugins/calpress-event-calendar/app/model/class-calp-settings.php on line 242
Very sad, because it seems to be a great plugin we'd have happily bought the Developer Pro version of.